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the reason it runs all the time is because its not reaching the correct temp.
this is a defrost problem, this happens a lot in Frigidaire freezers, first thing you need to do is remove the back panel in the in inside the freezer and take a hair dryer and defrost all the coils real good, then put the panel back on, then order this, its the defrost thermostat, these go all the time, you can order this at http://www.repairclinic.com/SSPartDetail.aspx?s=t-PLFH1779G53-%3d%3di1483781&PartID=1483781 and it is snapped to the top of the coils you defrost, inside the freezer, hope this helps , thanks Bryan
ok, that great you can test this, let the freezer start to cool with the back panel off, shut the door and wait about 1/2 hour then shut the refrigerator off and unplug the thermostat, test it for continuity, the thermostat has to be cold to test this so leave it on the coils, if it has continuity when its colds its good , the next thing to check is the heater, unplug this and test it for continuity, this runs under the coils, this should have continuity all the time, if both those parts check out change the timer, , these are the only three parts of the defrost system, hope this helps. you can click this blue link for all the diagrams and parts,
http://www.searspartsdirect.com/partsdirect/getModel!retrieveModelDetails.pd?modelNumber=PLFH1779GS3&productCategoryId=0142000&brandId=1428&brandDescription=FRIGIDAIRE&modelName=FREEZER&searchedModel=PLFH1779G53 thanks Bryan